Sinusoidal noise cancellation systems are active noise reduction systems that are used to reduce or cancel one or more sinusoidal noise components. Sinusoidal noise cancellation systems use one or more error microphones as input transducers. A reference signal related to the noise to be canceled (e.g., a sinusoid having a frequency component that corresponds to the noise to be reduced) is inputted to an adaptive filter. The output of the adaptive filter is applied to one or more transducers that produce sound (i.e., loudspeakers). In order to cancel the sinusoidal noise the output of the loudspeaker needs to be of equal magnitude and frequency but opposite phase to the sinusoidal noise at the error microphone location. The adaptive filter can alter the magnitude and/or the phase of the reference signal with the aim of converging the output to the sinusoidal noise at the error microphone so as to reduce the microphone signal to zero. The adaptive filter adaptively adjusts its internal filter coefficients so as to develop an output signal that is calculated to cancel the sinusoidal noise. The aim of the system is to cancel the microphone signal at the frequency or frequencies of interest.
Sinusoidal noise cancellation systems can be used in any situation in which it is desirable to cancel sinusoidal noise. Some applications include motor vehicles, where the systems are used to reduce or cancel sinusoidal (e.g., harmonic) noise in the vehicle cabin. The sources of noise can include noise produced from rotating devices such as the engine and the propeller (prop) shaft, which produce harmonics that can be desirable to cancel. Sources of sinusoidal noise in motor vehicles also include other rotating devices such as the air conditioning compressor or the tires, or non-rotating devices or noise sources such as resonance in the vehicle cabin resulting from vibration of cabin components, such as interior trim or the vehicle headliner. Another example of a non-rotating noise source could be noise resulting from air/wind passing through the vehicle cabin (e.g., via a vent or open window) or through the engine compartment.
In certain situations these sinusoidal noise cancellation systems can become unstable and allow the loudspeaker sound output levels that are designed to cancel the sinusoidal noise to diverge. Such an unstable sinusoidal noise cancellation system can produce loud and noticeable noise artifacts. One cause of such instability can be a change in the loudspeaker to error microphone transfer function(s).